MOVEMENT OF ANIMALS, i. 



it is on this account that animals have joints. For 

 they use their joints as a centre, and the whole part 

 in which the joint is situated is both one and two," 

 both straight and bent, changing potentially and 

 actually because of the joint. And when the part 

 is being bent and moved, one of the points in the 

 joint moves and one remains at rest, just as would 

 happen if A and D in the diameter of a circle were 

 to remain still while B moved, and the radius AC 

 were formed. (In geometrical figures, however, the 

 centre is considered to be 

 in every respect indivisible — 

 for movement, too, in such 

 figures is a figment, so they 

 say, since in mathematics 

 nothing actually moves, — 

 whereas the centres in the 

 joints are, potentially and 

 actually, sometimes one and 

 sometimes divided.) Be that as it may, the origin ^ 

 to which the movement can be traced, qua origin, 

 is always at rest while the part below it is in motion 

 — the elbow-joint, for instance, when the forearm 

 is in motion, the shoulder when the whole arm is 

 moved, the knee when the shin is moved, and the 

 hip when the whole leg is moved. It is obvious, then, 

 that every animal too must have in itself something 

 that is at rest, in order to provide that which is 

 moved with the origin of its movement, supported 



whether, as here, in a single member, or at the centre of the 

 body, viz. the heart (701 b 25, 29), where a further idea of 

 " ruling " seems to be implied {e.g. 703 a 37). It is also used 

 sometimes in the literal sense of " beginning," and this and 

 the meaning of " origin " of motion may occur in the same 

 passage and cause confusion {e.g. 702 a 36-b 2). 



P 443 



